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| United States Patent |
6,264,762
|
|
Bommer
, et al.
|
July 24, 2001
|
Corrosion resistant magnesium compositions and applications thereof
Abstract
A magnesium alloy material includes magnesium; more than 1 wt. % manganese;
and at least one sp-metal selected from the group consisting of zinc,
cadmium, mercury, gallium, indium, thallium, germanium, tin, lead,
arsenic, antimony, and bismuth, wherein the manganese and the at least one
sp-metal together are a maximum of 5 wt. % of the alloy material. The
magnesium materials are resistant to corrosion and are especially useful
in articles exposed to aqueous electrolytes during use or production.
| Inventors:
|
Bommer; Heike (Worth, DE);
Lang; Jurgen (Backnang, DE);
Nitschke; Felix (Munich, DE)
|
| Assignee:
|
DaimlerChrysler AG (Stuttgart, DE)
|
| Appl. No.:
|
934597 |
| Filed:
|
September 22, 1997 |
Foreign Application Priority Data
| Sep 21, 1996[DE] | 196 38 764 |
| Current U.S. Class: |
148/420; 148/666; 148/667; 420/411; 420/412; 420/413 |
| Intern'l Class: |
C22C 023/00 |
| Field of Search: |
420/411,412,413
148/420,666,667
|
References Cited
U.S. Patent Documents
| 1992655 | Feb., 1935 | Fischer | 75/1.
|
| 2270193 | Jan., 1942 | McDonald | 75/168.
|
| 3947268 | Mar., 1976 | Tikhonova et al. | 420/413.
|
| 4194908 | Mar., 1980 | Unsworth et al. | 75/168.
|
| 4332864 | Jun., 1982 | King et al. | 429/3.
|
| 5342576 | Aug., 1994 | Whitehead | 420/413.
|
| Foreign Patent Documents |
| 679 156 | Jul., 1939 | DE.
| |
| 1 939 794 | May., 1972 | DE.
| |
| 1433108 | Aug., 1993 | DE.
| |
| 923066 | Apr., 1963 | GB.
| |
| 1251223 | Oct., 1971 | GB.
| |
| 1601118 | Oct., 1981 | GB.
| |
| 62-218527 | Sep., 1987 | JP.
| |
| 6256883 | Sep., 1994 | JP.
| |
| 8020835 | Jan., 1996 | JP.
| |
| 461963 | Feb., 1975 | SU.
| |
| 1770431 | Oct., 1992 | SU.
| |
| 9315238 | Aug., 1993 | WO.
| |
Primary Examiner: Ip; Sikyin
Attorney, Agent or Firm: Evenson, McKeown, Edwards & Lenahan, P.L.L.C.
Claims
What is claimed is:
1. A magnesium alloy material, consisting of:
magnesium;
more than 2 wt. % manganese; and
a corrosion-resisting amount of at least one sp-metal selected from the
group consisting of cadmium, mercury, indium, thallium, germanium, tin,
lead, arsenic, antimony, and bismuth,
wherein the manganese and the at least one sp-metal together are a maximum
of 5 wt. % of the alloy material.
2. A magnesium alloy material according to claim 1, comprising from 0.1 to
2 wt. % of the at least one sp-metal.
3. A magnesium alloy material according to claim 1, comprising from 0.1 to
1 wt. % of the at least one sp-metal.
4. A magnesium alloy material according to claim 1, wherein at least one of
manganese and the sp-metal has an exchange current density for hydrogen
reduction of no more than 10.sup.-7 A/m.sup.2.
5. A magnesium alloy material according to claim 2, wherein at least one of
manganese and the sp-metal has an exchange current density for hydrogen
reduction of no more than 10.sup.-7 A/m.sup.2.
6. A magnesium alloy material according to claim 3, wherein at least one of
manganese and the sp-metal has an exchange current density for hydrogen
reduction of no more than 10.sup.-7 A/m.sup.2.
7. A magnesium alloy material according to claim 1, wherein there are no or
substantially no intermetallic bonds.
8. A magnesium alloy material according to claim 2, wherein there are no or
substantially no intermetallic bonds.
9. A magnesium alloy material according to claim 3, wherein there are no or
substantially no intermetallic bonds.
10. An article of manufacture, which comprises a component comprising a
magnesium alloy material according to claim 1.
11. An article of manufacture, which comprises a component comprising a
magnesium alloy material according to claim 2.
12. An article of manufacture, which comprises a component comprising a
magnesium alloy material according to claim 3.
13. A method of making a metallic part resistant to corrosion upon exposure
to aqueous electrolytes comprising producing said part from a magnesium
alloy material according to claim 1.
14. A method of making a metallic part resistant to corrosion upon exposure
to aqueous electrolytes comprising producing said part from a magnesium
alloy material according to claim 2.
15. A method of making a metallic part resistant to corrosion upon exposure
to aqueous electrolytes comprising producing said part from a magnesium
alloy material according to claim 3.
16. A magnesium alloy material according to claim 1, wherein the at least
one sp-metal is tin.
17. A magnesium alloy material according to claim 1, wherein the at least
one sp-metal is indium.
18. A magnesium alloy material, consisting of:
magnesium;
more than 1 wt. % manganese; and
at least one of tin, arsenic, or antimony,
wherein the manganese and the at least one of tin, arsenic, or antimony are
a maximum of 5 wt. % of the alloy material.
Description
BACKGROUND AND SUMMARY OF THE INVENTION
This application claims priority of German Patent Application 196 38 764.7,
filed Sep. 21, 1996, the entire contents of which are incorporated herein
by reference and can be relied on to practice the claimed invention.
Materials containing magnesium are important for lightweight construction
in a number of fields. For example, lightweight magnesium compositions are
employed in a variety of parts in the automobile industry, in engine
construction technology, in aerospace technology, and in other structural,
lightweight objects in the computer industry and the domestic appliance
industry. The low specific weight of magnesium and its good strength
characteristics allow a considerable weight reduction in components in
comparison to parts made of aluminum or steel. The better pourability of
magnesium alloys, in comparison to materials made of aluminum, also
results in a reduction in processing steps and an increase in
productivity. And, in contrast to materials made of aluminum, complicated,
thin-walled parts can be made in large numbers by casting. Using materials
made of magnesium in transport means also opens up a potential for
lowering costs, saving fuel, and increasing payload.
The energy required for the primary production of magnesium competes with
the price of primary production of aluminum. When magnesium is recycled,
only 5% of the recovered energy is required to recycle it. This represents
an improved overall energy balance for recycling materials made of
magnesium by comparison to those made of aluminum. However, even if no
recycling is performed, materials made of magnesium can readily be
returned to the resource cycle in nature.
Today only a few percent of lightweight construction applications are made
from magnesium alloys. Therefore, the use of magnesium compositions with
the highest specific strength and the highest specific modulus of
elasticity offers a high potential for increasing production economy and
reducing environmental impacts.
However, an obstacle to using alloys made of magnesium is their corrosion
behavior. Corrosion media that contains water can considerably affect the
function of magnesium parts. In order to improve the corrosion resistance
of magnesium parts, it is known to give them so-called conversion layers,
especially conversion layers in which chromate (VI) ions are embedded in
the surface of the part. Magnesium parts are also anodized. Both
conversion layers and the anodization of parts, however, merely result in
the passivity of the surface. This means that if the passivated surface
layer is damaged, the corrosion protection fails at the damaged point.
The addition of cathodic impurities into materials made of magnesium can
never be avoided completely. The quantity of cathodic precipitates has
been reduced to a minimum since the development of highly pure magnesium
alloys, but because of the manufacturing process, these precipitates are
frequently present on the surface.
A goal of the invention is to provide a magnesium composition and/or
material that has a high level of corrosion resistance in aqueous
electrolytes. This is accomplished according to the invention with a
magnesium composition or material that contains at least one of the
elements from the group composed of sp-metals and manganese. The term "sp
metals" refers to those metals whose outer s- or p-states of electron
configuration are not filled.
In particular, the sp-metals consist of Zn, Cd, Hg, Ga, In, Tl, Ge, Sn, Pb,
As, Sb, and Bi. The sp metals and/or manganese together account for a
maximum of 5 wt. of the magnesium composition or material. Preferably,
however, the content of these metal elements is only 0.1 to 2 wt., and
especially 0.2 to 1 wt., since at higher concentrations intermetallic
bonds can develop, which, as will be explained below, do not possess any
corrosion-resisting properties, or in any event have reduced corrosion
resisting properties.
The invention comprises, more specifically, a magnesium material
characterized by a content of up to 5% by weight of at least one element
from the group of sp-metals and manganese. Preferably, the content of the
at least one element amounts to from 0.1 to 1% by weight. As noted, the
sp-metal can be zinc, cadmium, mercury, gallium, indium, thallium,
germanium, tin, lead, arsenic, antimony or bismuth, or a combination of
metals.
Generally, the magnesium material is characterized in that the at least one
element has an exchange current density for hydrogen reduction of no more
than 10.sup.-7 A/m.sup.2. Also, the magnesium material generally contains
the sp-metal and/or manganese element(s) without the formation of
intermetallic bonds or compounds, or substantially without intermetallic
bonds or compounds. The invention also comprises the use of a magnesium
material in a component that is exposed to aqueous electrolytes and
methods for making corrosion-resistant parts.
DETAILED DESCRIPTION OF THE INVENTION
In general, magnesium materials can be pure magnesium metal or a magnesium
alloy, especially a commercially available magnesium alloy.
The corrosion of magnesium metal in an aqueous medium proceeds according to
the following reaction equations:
##EQU1##
Reaction (1) represents anodic oxidation while the reaction equations (2a)
and (2b) are the reduction reaction or adsorption reaction according to
Volmer. This means that according to (H.sub.at) and these are then
adsorbed (H.sub.ad). The recombination of two H.sub.ad to molecular
hydrogen proceeds according to the so-called Tafel or Heyrowsky reaction.
During the corrosion of magnesium parts, the reduction reaction proceeds
especially at those points where, for example, a less negative potential
is present as a result, for example, of cathodic impurities in a magnesium
alloy. These cathodic impurities can be present as more noble metals,
copper, iron, or nickel, for example, or a higher aluminum content. At
these cathodic points on the part, partial reactions (2a) and (2b),
hydrogen development, takes place.
The elements used in the compositions and materials according to this
invention are characterized in that they result in a high hydrogen
overpotential, in other words they inhibit both hydrogen reduction (2a)
and hydrogen adsorption (2b). By virtue of their homogeneous distribution
in the mixed crystal, these elements influence the electron configuration
of the matrix and poison, so to speak, the development of hydrogen so that
the anodic oxidation of the magnesium according to equation (1) does not
take place.
The high hydrogen overpotential of these elements expresses itself in a
correspondingly low exchange flow density for hydrogen reduction and this
exchange flow density for these metals is no more than 10.sub.-7
A/m.sup.2.
The elements should be distributed as homogeneously as possible in the
magnesium material of this invention. In no event may intermetallic bonds
form from these elements since they have different properties, especially
a different exchange current density for hydrogen reduction, and thus will
not lead to the desired high hydrogen overpotential.
The formation of intermetallic bonds can be prevented by adding as little
as possible of the alloy elements or by appropriate manufacturing methods
for the magnesium material. For example, formation of intermetallic bonds
can be prevented by using powder metallurgy methods, using magnesium or
magnesium alloy powder, and an alloy element powder, or heat treatment
with rapid cooling. Other methods are also known the art.
In addition to their intervention in partial reactions (2a) and (2b), the
sp-metal or manganese elements act as anodes when the magnesium part, as a
result of impurities, for example, contains locations with a less negative
potential acting as cathodes.
When the sp-metals and manganese are oxidized as anodes, they form
compounds that are difficult to dissolve, namely oxides and/or hydroxides.
In addition, intermediate compounds such as chlorides can form first,
which then are hydrolyzed in the alkaline marginal area of the surface of
the part, especially at the cathodic reaction locations. As a result of
their anodic oxidation to form the above mentioned compounds, the sp
metals and/or manganese stop corrosion and cure the material.
As is apparent from the above to one skilled in the art, the magnesium
compositions or material according to the invention are especially
suitable for parts that are used in aqueous electrolytes. The
corrosion-resistance effect of the sp metals or manganese is thus observed
in both halide-containing and halide-free aqueous media, and also at
elevated temperatures. Since the invention achieves a corrosion resistance
independent of the state of the surface, a highly reliable
magnesium-containing part results.
EXAMPLE
A magnesium alloy material containing 3% manganese was made as follows:
Ninety-seven (97) parts by weight of ZC63 (a commercial magnesium alloy
containing 6% Zn and 3% Cu) and 5.88 parts by weight of MnCl.sub.2 are
heated up to 700-720.degree. C., in an inert gas atmosphere (argon with
SF.sub.6). The system is maintained at this temperature for 30 minutes for
sedimentation and for reaction of MnCl.sub.2 to Mn and Cl.sub.2. The melt
is filled in molds under pressure. A gooseneck is required for this
process to separate the contaminations of a higher density, such as those
containing iron. Then, the melt cools down to 100.degree. C. in 90
seconds. Further cooling takes place under air at room temperature. The
magnesium alloy contains 3% Mn. However, similar conditions can be used,
as would be apparent to one skilled in the art from this disclosure, to
produce magnesium alloys of up to 5 wt. % Mn, or in specific ranges, such
as 0.1 to 2 wt. % or 0.1 to 1 wt. %.
Characterization of the Corrosion-resistant Properties of ZCp63+3% Mn
Compared with Mg Pure and ZC63
The corrosion behavior of the Mg-materials were investigated with an
alternate immersion test (ASTM-Standards G-44).
The conditions were as follows:
Solution: 3.5 mass-% NaCl, pH 7
Volume of Solution: 32 ml/cm.sup.3
Test-duration: 4 h
Immersion duration: 50 min
Relative Humidity: 45% +/- 10%
Temperature: 27.degree. C. +/- 1.degree. C.
The presence or attachment of corrosion is estimated on transversal
polishings with metallographic methods. The results are shown on the table
below.
Maximal deep of number of
Alloy pittings [.mu.m] pittings/cm
Mg.sup.+ 400 222 (total attack)
ZC63* 220 46
ZC63 + 3% Mn** 30 30
*comparison
**invention
Dramatic reductions in the depth of pitting and in the number of pittings,
a measure of the progress and occurrence of corrosion, can be seen with
the composition of the invention when compared to prior compositions.
Although the invention has been described and illustrated in detail, it is
to be clearly understood that the same is by way of illustration and
example, and is not to be taken as a limitation. The spirit and scope of
the present invention are to be limited only by the terms of the appended
claims.
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